This invention relates to soft, flexible, microporous films having high tensile strength and good air and water vapor transmission rates therethrough but which are substantially impenetrable to liquid water. This invention also comprehends a process for making such films by stretching certain highly filled polymer compositions under controlled conditions.
Prior to the present invention, breathable microporous film were made by a variety of techniques from polymers containing solid substances dispersed therein. Sometimes the pores of the film were obtained by dissolving or leaching out those dispersed materials (fillers) and sometimes pores were formed when the filled polymer material was stretched. Often the porous films produced by stretching the filled polymer compositions were stiff, low tear strength products, irrespective of the type of matrix polymer used. Hence, there is a need in the hygienic product industry for liquid barrier yet breathable, disposable products such as in feminine panty liners, diapers, bed sheets, and hospital gowns. In order for public acceptance of breathable products, the user should be cool and comfortable and the product has to be soft and flexible without irritating the skin of the user. There is also a need for such breathable films in the battery industry for use as battery separators. The present invention solves the above-mentioned needs.
U.S. Pat. No. 4,698,372 discloses a microporous polymeric film having good water vapor transmission rates and hydrostatic resistance to water penetration thereof; the film has a filler loading of about 25-35 volume % of inorganic fillers such as calcium carbonate, among others, and uses an "antagonizer" such as stearic acid in order to reduce the effective surface tension of the filler to the approximate level of that of the matrix polymer. U.S. Pat. No. 3,903,234 discloses gas permeable biaxially oriented film prepared from compositions of polyolefins containing 26-50% by weight of inorganic filler particles. U.S. Pat. No. 4,176,148 discloses microporous oriented films composed of polybutene containing 3-80% by weight of inorganic fillers. British Patent 2,151,539 discloses a process for making water vapor permeable oriented film from polyolefins containing 33-83% by weight of barium sulfate filler.
None of the above prior references discloses the instant invention.